We have been studying gene expression in cytotoxic immune cells with specific emphasis on large granular lymphocytes (LGLs) and cytotoxic T cells. We have been characterizing a gene (NK-TR) which appears to function in the killing mechanism of LGLs and is also expressed in the cytoplasm of T cells, a rat natural killer (NK) cell line and a human T- cell line with NK-killing activity. NK TR has also been found to be induced in differentiating peripheral blood monocytes. These studies suggest a crucial role for NK-TR in the generation of competent immune effector cells.